The present invention is directed, in general, to wireless networks and, more specifically, to a system for performing a handoff between base stations of a mobile station that is in the process of accessing the wireless network.
Wireless subscribers use a wide variety of wireless devices, including cellular phones, personal communication services (PCS) devices, and wireless modem-equipped personal computer (PCs), among others. The large number of subscribers and the many applications for wireless communications have created a heavy subscriber demand for RF bandwidth. To maximize usage of the available bandwidth, a number of multiple access technologies have been implemented to allow more than one subscriber to communicate simultaneously with each base station (BS) in a wireless system. These multiple access technologies include time division multiple access (TDMA), frequency division multiple access (FDMA), and code division multiple access (CDMA). These technologies assign each system subscriber to a specific traffic channel that transmits and receives subscriber voice/data signals via a selected time slot, a selected frequency, a selected unique code, or a combination thereof.
As is well known, CDMA systems divide the RF spectrum into a number of wideband digital radio signals. Each digital radio signal carries several different coded xe2x80x9cchannelsxe2x80x9d. Each coded channel is distinguished by a unique pseudo-random noise (PN) code used by the mobile station and/or the base station. In a CDMA receiver, the coded channels are decoded by a signal correlator that matches PN sequences. Some coded channels are used as data traffic channels to transport subscriber voice and/or data signals, while other coded channels are used as control channels, including pilot, synchronization, paging and access channels.
When a mobile station accesses a base station in a CDMA wireless network, the CDMA control channel (which has one unique PN code) assigns the mobile station to a data traffic channel (which has a different PN code) on which the mobile station exchanges data traffic with another party, a data terminal, a fax machine, or the like. Typically, the coded control channel and the data traffic channel(s) used by the mobile station and the base station are on the same RF carrier frequency. Advantageously, in many CDMA networks, the control channels and the data traffic channels are on the same RF carrier frequency in all cells (i.e., base station coverage areas) in the networks.
Because adjoining base stations in a CDMA wireless network may operate on the same RF carrier frequency, a mobile station can simultaneously access two or more base stations by using the same PN codes to communicate in data traffic channels and control channels with each base station. This enables a process know as xe2x80x9csoft handoffxe2x80x9d to occur when a mobile station communicating with a first base station enters a second base station""s cell site. A soft handoff may occur while a mobile station is actively communicating with another device (e.g., a voice phone call). A soft handoff also may occur when the mobile station is turned xe2x80x9cONxe2x80x9d and has accessed the CDMA network, but is not actively communicating with another device. This is known as an xe2x80x9cidlexe2x80x9d handoff.
Field data on CDMA wireless networks have shown a high percentage of call drops experienced by mobile stations in the system access state. In a CDMA wireless network, call setup failures frequently occur due to very quick propagation environment changes, such as when a mobile station moves behind a hill or a large building. This can cause problems when the mobile station is in the middle of a system access operation. In older systems, when a mobile station sends an Origination message to a first base station or receives a Page message from that first base station, the mobile station enters a xe2x80x9csystem accessxe2x80x9d state during which the mobile station cannot undergo a handoff to a second base station. If the RF propagation environment changes quickly enough, the mobile station may not receive a channel assignment message from the first base station and the data traffic channel will not be established. More recently, technological solutions to this problem have been proposed in the CDMA air interface by providing mobile stations with the ability to perform soft handoff operations during system access states with cell sites having strong pilot signals.
However, when a mobile station enters a system access state and attempts to access the CDMA wireless network through more than one base station (BS), a related problem arises in the interface between the bases stations and the mobile switching center (MSC). To initiate a system access, a mobile station sends an Origination message to a base station, which responds with BS Acknowledgment Order message. However, due to the quick changes in the RF propagation environment described above, the mobile station may not receive the BS Acknowledgment Order message. The mobile station may then send an Origination message to another base station. As a result, the mobile station may send Origination messages to several base stations, each of which in turn sends a CM Service Request (CMSR) message to the mobile switching center.
Unfortunately, the MSC has no way of knowing with which base station the mobile stations is currently in communication, or is the mobile station has been or is being handed-off from a first base station to a second base station during the access probe operation. Currently there are two approaches proposed to support access probe handoff over the MSC-BS interface.
One approach requires the MSC to respond to all the CMSR messages it receives via different base stations from a single mobile station for the same call. Thus, the;MSC sends multiple Assignment Request messages to assign multiple resources to set up one call. This results in wasted/idle network resources, dropped calls, and potential reduction in capacity. Network resources are wasted or idle because only one of the assigned resources ultimately is used to establish the call, while the others are not used. Calls may be dropped because the multiple resources setup in the public switched telephone network (PSTN) for the same call may result in network errors.
The second approach requires the MSC to respond to the first CMSR message it receives and to ignore the rest. Since the MSC responds only to the first CMSR message, there is a significant chance for the call to be dropped. The MSC will likely receive the first CMSR message from the first base station the mobile station attempts to access. By the time the base station receives the Assignment Request message from the MSC, the mobile station may have moved out of the first base station""s coverage area and the call will be dropped.
There is therefore a need in the art for improved CDMA wireless network systems that are less likely to drop calls from a mobile station during access probe handoff operations. In particular there is a need for an improved BS-MSC interface that uses less network resources to handle an access probe handoff operation and transmits fewer call set-up messages across the wired backbone between the base stations and the mobile switching center of a wireless network.
The present invention discloses systems and methods that support access probe handoff over the interface between the mobile switching center (MSC) and the base station (BS). The disclosed procedure allows for efficient use of the signaling link between the MSC and the base stations by minimizing the number of signaling messages required to successfully perform an access probe handoff. The disclosed procedure also increases the reliability of the access probe handoff.
To address the above-discussed deficiencies of the prior art, it is a primary object of the present invention to provide, for use in a wireless network comprising a plurality of base stations capable of communicating with a plurality of mobile devices, an apparatus for establishing a call connection between a first one of the plurality of mobile devices and one of the plurality of base stations. The apparatus comprises a message control processor capable of receiving from the plurality of base stations cell information messages generated by the first mobile station, each of the cell information messages comprising cell identification (ID) data identifying ones of the plurality of base stations from which the first mobile station is receiving wireless messages. The message control processor, in response to receipt of a first cell information message from a first base station and receipt of a subsequent second cell information message from a second base station, processes only one of the first and second cell information messages and transmits to only one of the first and second base stations a resource allocation message capable of establishing the call connection with the first mobile station.
The xe2x80x9ccell information messagesxe2x80x9d may be Paging Response messages or CM Service Request messages transmitted by a mobile station (via one or more base stations) to the mobile switching center (MSC). The xe2x80x9cresource allocation messagexe2x80x9d may be an Assignment Request message sent from the MSC to a base station allocating radio resources in the base station to communicate with the mobile station.
According to an advantageous embodiment of the present invention, the apparatus is disposed in a mobile switching center (MSC) of the wireless network. This central location allow the message control processor to operate in an efficient manner. However, it is not absolutely required that the apparatus be disposed in the MSC. In alternate embodiments of the present invention, the message control processor may be a stand-alone device that communicates with the base stations and the MSC, or the message control processor may be disposed in one of the base stations. Situating the message control processor outside of the MSC requires additional message transfers over the wired backbone of the wireless network that are not required when the message control processor is disposed in the MSC.
According to one embodiment of the present invention, the message control processor compares first cell ID data of the first cell information message and second cell ID data of the second cell information message, and, if the second cell ID data is the same as the first cell ID data, the message control processor processes the first cell information message and transmits the resource allocation message to the first base station.
According to another embodiment of the present invention, the message control processor compares first cell ID data of the first cell information message and second cell ID data of the second cell information message, and, if the second cell ID data is different than the first cell ID data, the message control processor processes the second cell information message and transmits the resource allocation message to the second base station.
According to yet another embodiment of the present invention, the apparatus further comprises a timer associated with the message control processor capable of measuring a time period, T, after receipt of the first cell information message, wherein the message control processor processes only the first cell information messages and transmits the resource allocation message only to the first base station if the second cell information message is received after the time period, T, expires.
According to still another embodiment of the present invention, the message control processor compares first cell ID data of the first cell information message and second cell ID data of the second cell information message, and, if the second cell ID data is the same as the first cell ID data, the message control processor continues processing the first cell information message and ignores the second cell information message.
According to a further embodiment of the present invention, the message control processor compares first cell ID data of the first cell information message and second cell ID data of the second cell information message, and, if the second cell ID data is different than the first cell ID data, the message control processor stops processing the first cell information message and begins processing the second cell information message.
The present invention has performance advantages over the previous approaches. Since the MSC sends only one Assignment Request message, only one set of network resources are assigned for each call made by any given mobile station. Additionally, the disclosed procedure also allows resources to be assigned in a base station that has a comparatively high probability of successfully communicating with the mobile station.
The foregoing has outlined rather broadly the features and technical advantages of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features and advantages of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they may readily use the conception and the specific embodiment disclosed as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form.
Before undertaking the DETAILED DESCRIPTION, it may be advantageous to set forth definitions of certain words and phrases used throughout this patent document: the terms xe2x80x9cincludexe2x80x9d and xe2x80x9ccomprise,xe2x80x9d as well as derivatives thereof, mean inclusion without limitation; the term xe2x80x9cor,xe2x80x9d is inclusive, meaning and/or; the phrases xe2x80x9cassociated withxe2x80x9d and xe2x80x9cassociated therewith,xe2x80x9d as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like; and the term xe2x80x9ccontrollerxe2x80x9d means any device, system or part thereof that controls at least one operation, such a device may be implemented in hardware, firmware or software, or some combination of at least two of the same. It should be noted that the functionality associated with any particular controller may be centralized or distributed, whether locally or remotely. Definitions for certain words and phrases are provided throughout this patent document, those of ordinary skill in the art should understand that in many, if not most instances, such definitions apply to prior, as well as future uses of such defined words and phrases.